The present invention relates to electromagnetic wave shielding materials, more particularly to a copper powder of which surface has been coated with an metal organic compound and to an electroconductive paint composition for shielding electromagnetic wave which contains the copper powder.
One of the electromagnetic wave shielding materials for the purpose of protecting electronic equipments from disturbance or jamming by electromagnetic waves is a electroconductive paint in which an electroconductive filler such as nickel powder, silver powder, copper powder or carbon powder is kneaded with any of various resin binders. Shielding against electromagnetic waves is accomplished by applying the paint on the surface of plastic molded articles by spraying or painting. Electroconductive paints of copper type are inexpensive in comparison with that of silver or nickel types and have excellent shielding effect.
However, in the electroconductive paints of copper type, a uniform dispersion state cannot be realized because the copper particles tend to agglomerate together in the paint. Moreover, the electroconductive paints of copper type tend to be easily oxidized by environmental factors such as heat and humidity, which leads to inferior storage stability and the deterioration of resistance to aging (attenuation of shielding effect). Many methods have hitherto been proposed for the purpose of solving these problems. The methods include the treatment of the particles of electrolytic copper powder with an organic carboxylic acid (Japanese Patent Laid-Open Application No. 258273/85), the surface treatment of copper powder with a coupling agent (Japanese Patent Laid-Open Application No. 30200/85), coating of the particles of electrolytic copper powder with silver (Japanese Patent Laid-Open Application No. 243277/85), coating of the particles of copper powder with an organic titanate (Japanese Patent Laid-Open Application Nos. 174661/84 and 36553/81), coating of the particles of copper powder with an organic aluminum (Japanese Patent Laid-Open Application No. 179671/84), metal plating of copper powder with solder (Japanese Patent Laid-Open Application No. 113505/82), coating of the particles of copper powder with copper oxide (Japanese Patent Laid-Open Application No. 35405/85) and the like. The copper powder coated with the above described surface coating agent has some advantageous effects.
However, the rust prevention of copper powder is not good, and the electroconductivity and environmental resistance of copper powder are insufficient. Moreover, conductive paints obtained with copper powder in the prior art, when used by the application thereof onto a matrix or base material, are inferior in adhesion to the matrix and do not always exhibit excellent storage and environmental resistance.